I use the JLC stainless stencils which I find quite good. The difficulty I have is getting paste on tiny pads like U-DFN and for other truely “leadless” parts. I have fresh paste which I keep cold, then quickly apply and stick in the PnP. The amount of paste is so small it can’t have any thermal mass. I’ve considered putting the stencil in the fridge (and pcb) but then moisture condenses on it immediately. So that’s not a good solution.
I would recommend that you look at the “Type” of paste that you have. In addition to the solder composition, there is also a type, usually denoted T3, T4, T5 and this represents the size of the balls of solder suspended in the paste. “Type” is like wire gauge, the bigger the number, the smaller the balls.
I think that T3 is pretty typical paste but for larger geometries. For small apertures like DFNs or BGAs, the balls get jammed in the stencil and don’t stick to the pad. The also plug the holes for future wipes.
You might be using T3 paste…
I believe that you want to have T4 paste and this will make a big difference.
You might say “what the heck, why not be safe and jump to T5”. I thought the same thing until I purchased T5 and tried it. What we found with T5 paste is that it has a much more “runny” consistency. Typical paste sort of holds a rectangular shape when you pull the stencil off. T5 tends to flatten and ooze onto adjacent pads.
The paste is cheap enough that you could order both and see what works for you but after about 2 months worth of experiments, we did arrive at T4 being the “Goldilocks” paste.
When I apply the paste cold, I find that it doesn’t want to stick to anything. So I typically give my paste some time to warm up before using, but for smaller stuff, perhaps that would make the paste too runny for you. +1 for the recommendation to look into smaller ball sizes. My go-to is chipquick SMD291AX in T3 (I also use JLC stencils) but the smallest pad I’ve done with it was probably an SOT-723 which has something like .22mm x .22mm pads.
What do folks use for PCB wash prior to applying paste? I’m doing hand work but I wipe the boards with alcohol on a clean cloth just to degrease.
I have T4 size paste. My “process” is align the stencil then use an old credit or hotel key card to squeegee the paste. I try to only make 1 pass and press it down firmly. More passes just forces the paste under the stencil. A second light pressure pass cleans off the excess on the stencil.
I typically do not wash prior to stencil unless I’m wiping off old paste. If I do wash it is always with IPA.
I used to be a 1 wipe kind of person but now I’m ok with swiping a few times if there was a bubble or something preventing a good print. I do fine that with small boards, it is more difficult to keep the stencil planar with the PCB. I definitely prefer doing larger panels of PCBs.
I also try to have at slug of paste that is at least the diameter of a pencil on the edge of the card. You want that paste to “roll” as you wipe with some very firm pressure on the edge.
I don’t gt too concerned if there is extra coat of paste on the stencil before lifting. Ideally not but I’ve had worse results trying to clean it up than just lifting the stencil anyway.
Slightly off topic: I’ve seen those small stencils for just 1 chip. Not sure if they are DIY by cutting up a larger stencil or a purchased item. They have sides bent up and a stick to hold them. This is for QFN and various SMD footprints.
For phone repair technicians, you can buy a stencil variety pack off aliexpress and then cut out the desired stencil as needed – the stencils are obviously specific to each board.
You can use OSHstencils.com (US based) or any of the many Chinese PCB houses to get your own stencils, of course. When I order boards from PCBWay, if there’s anything non-trivial, I just add a stencil to my order for $10 – sometimes the stencils will delay the production time by a day with them, as the stencils seem to take longer than the boards.